To give a brief introduction to hypermedia we present an example
here showing how an imaginary hypermedia system works. The tour
metaphor is frequently used in connection with hypermedia, as
hypermedia permits "travelling" in an abstract information
space. This example is therefore a brief "tour" through
hypermedia. The example shows a document, called "Hypermedia",
showing the hypermedia components: links, nodes and diagrams.
Hypermedia is a network containing several interlinked information
units. The information units are hereafter called nodes, and the
connections between the nodes are called links. Limited networks
of nodes and links are called hyperdocuments or simply documents.
Figure 1.2 shows a typical node. The window is the node itself,
and in this case the name of the window is the name of the hyperdocument.
Inside the window (the node) there are links and information objects.
This node holds only one type of information object, text objects.
Two types of link are represented in the node, reference links
and organisation links.
Organisation links handle the structure of the hyperdocument, analogous to chapters and paragraphs in an ordinary printed text. Reference links are occasional references to other material, e.g. bibliography references and footnotes. How links are indicated normally varies from system to system. In the example above reference links are indicated using a little circle in front of the link word, and the link words of the reference links are underlined.
The screen in Figure 1.1 allows us firstly to read the information
shown in the window, secondly to move on to some other information.
Assume that in the next step we choose to look more closely at
the topic "links", clicking the indicated spot "links"
with the mouse. This is a reference link to another topic, and
thus we cross the original structure of the document. In effect
this is like moving on to one of the references when we look up
something in an encyclopaedia. The indicated spot is often called
the link spot. The new node which deals with links appear, shown
in Figure 1.3. This is a new window with new information and new
links.
Similarly a user may follow a path through a network of information.
Users often select links spontaneously, hence information databases
are often accessed more intuitively than traditional databases,
which are used for queries using unique keys and value sets. Hereafter
this way of moving around in the database will be called "following
links". In the case above we followed a reference link, which
is a random link. If we had chosen one of the organisation links
we would have followed the author's primary document structure.
In larger hyperdocuments and hypersystems a common problem is
that the user loses track of his/her whereabouts in the document,
i.e. the user cannot manage to extract the correct information
from the database. One of a number of possible solutions to this
problem is to present the hyperdocument structure or a selected
part of it in a graphic overview or database diagram.
One possible way of structuring such a database diagram is shown
in Figure 1.4. Each node is represented as a box bearing the node
name, and the links are indicated by lines between the boxes.
The hyperdocument structure is rendered graphically, allowing
the user to see its structure. The example shows only the organisation
links, displaying the organisational structure. Other types of
organisation charts may show reference links, displaying the reference
structure, if any. Often the open (active) node is indicated,
permitting the user to see his or her present location in the
document. In this case the node "links" is open, thus
being shown in the colour black in the diagram. In most systems
a user may access a node directly by clicking on its box.
The above has been a brief examination of two methods for hypermedia
"navigation" via database diagrams and by following
links. Other methods, such as using a search engine and a programming
interface may also be used. Some of these methods will be examined
more closely later.